The present invention relates to a Micro-electronic-mechanical System (MEMS) scan controller generating clock frequency and a control method thereof, especially to a controller for micro-electric-mechanical mirrors (MEMS mirror) applied to bi-direction laser scanning units (LSU) and a control method that generate clock frequency signals so as to make a laser light source transmit laser beams within an effective scanning window according to the clock frequency signals.
Most of LSU available now uses a polygonal mirror rotating at high speed to control reflection direction of laser beam. However, due to hydraulic driving, working rotational speed limits, high manufacturing cost, high noises and delayed initiation, such LSU is unable to meet requirements of high speed and high precision by using polygon mirror. In recent years, MEMS mirrors with torsion oscillators are getting known and are going to be applied to LSU of imaging systems, scanners or laser printers in future. The MEMS oscillatory mirror developed based on principle of torsion oscillators has higher scanning efficiency than conventional polygon mirror.
In a laser scanning unit (LSU), a Micro Electronic Mechanical System (MEMS) oscillating mirror mainly consists of a control board with bridge circuit, a torsion oscillator and a mirror. A mirror driven by resonance magnetic field symmetrically oscillates along an axis. When a laser light is emitted to the mirror of the MEMS mirror, the MEMS oscillating mirror reflects the incident laser beam to the axis of the MEMS mirror at different angles for scanning along with different reflecting angles of the mirror surface that changes with time. Since the MEMS mirror scanning approach can neglect the wavelength effects, that the MEMS mirror has features of high resolution and large rotation angle so that has been applied broadly to commercial products, science and industries, such as devices disclosed in U.S. Pat. Nos. 5,408352, 5,867,297, 6,947,189, 7,190,499, TW Patent M253133 and JP 2006-201350. In order to improve scanning efficiency, a bi-directional LSU is developed yet associated control difficulties are raised.
Due to resonant oscillation of the MEMS mirror, the rotation angles and stability of the MEMS have effects on precision of the LSU. In a controller for bi-directional LSU of the MEMS mirror, conventional technique focuses on stability control of the MEMS mirror such as adjustment of resonant frequency, working angle, or by means of a voltage controlled oscillator (VCO) to control the frequency. The frequency control of the VOC is based on changing permittivity of dielectric material by current or change of the capacitance by the voltage, as shown in US2006/00139113, US2005/0139678, US2007/0041068, US2004/0119002, U.S. Pat. Nos. 7,304,411, 5,121,138, and JP63-314965. Take a bi-directional LSU with 600 dots-per-inch (dpi) resolution per A4 size as an example, 5102 light spots are sent per each scanning in one directional. The 5102 light spots are sent completely during an imaging interval while the imaging interval should be invariant with the frequency or amplitude variations of the MEMS mirror that lead to deviation of the light spot and the image is not formed on the object. Thus the calculation frequency of the MEMS mirror for sending correct signal to the laser controller that emits laser light is a main point of control. Refer to US2006/0279364, a method for determining an operating point of an oscillation controller is disclosed. A table derived from a model built by the method is used to define the operating point. The operating point may be expressed in terms of clock counts by factoring in the clock rate of the oscillation controller. Refer to U.S. Pat. No. 6,891,572, an interpolation circuit writes a video signal into a frame memory in synchronism with the write system clock from the PLL circuit. Refer to U.S. Pat. No. 6,838,661, a torsion oscillator is stabilized in operation by a PD detector. Refer to U.S. Pat. No. 6,870,560 and U.S. Pat. No. 6,987,595, rotation of drum and laser scan frequency are controlled by a counter controller or dynamic adjustment of resonant frequency. As to bi-directional scanning, in order to let the scanning beam not deviated and an image is formed on the object within the scanning window, a more fast and effective method is required.